In typical prior art disk drive servos, a microprocessor uses an estimator to predict position, velocity and bias force of an actuator. The actuator is typically a voice coil motor (VCM) driven by a power amplifier. This type of prediction is referred to as predicting the actuator dynamic. U.S. Pat. No. 4,697,127 (Stich et al.) discloses a seeking discrete estimator model which is formed by using a linear double integrator continuous model. In the servo of Stich et al., it is assumed that the actuator acceleration, which is related to the VCM current, is proportional to the input control signal. In order to ensure seek reliability, the power amplifier must match the estimator model and operate in the linear mode. This degrades the access time because the servo does not use all of the available power from the supply voltage.
U.S. Pat. No. 4,679,103 (Workman) discloses a disk drive servo that operates in the saturated mode. The acceleration is a function of the maximum drive voltage, and the VCM electrical characteristics when the power amplifier operates in the saturated mode. In order to operate in the saturated mode, the servo system measures the VCM current through an analog to digital (A/D) converter and uses it as the input to the estimator model. This increases the hardware cost which is the most important constraint in the current disk drive market.
U.S. Pat. No. 4,914,644 (Chen et al.) discloses a disk drive servo system that uses a saturated estimator model which includes the back electromotive force (emf) of the VCM and the coil current rise time. It uses the velocity error signal to determine whether to use the saturated or the linear model in the servo calculation. These two models are independently derived off-line by first forming continuous models and then converting them into discrete models.
One of the disadvantages of the foregoing technique is that it is quite complex and therefore is not normally used in simple disk drive servos. Such servos use the simplified linear estimator model. Another disadvantage of the prior art saturated technique is that it is not suitable for adaptively compensating for the back emf as a function of the VCM torque constant because the torque constant appears in most of the discrete time model coefficients.
U.S. Pat. No. 4,697,127 (Stich et al.) and U.S. Pat. No. 4,835,633 (Edel et al.) disclose techniques which identify the overall servo gain and adaptively use it in the servo calculation. Overall servo gain is defined as a factor which relates to the actuator acceleration. This factor represents a total servo channel gain variation which includes the power amplifier gain, the actuator torque constant and the actuator inertia. For a servo system which uses Pulse Width Modulation (PWM) for the digital to analog (D/A) converter, the overall servo gain also includes the power supply variation. The overall servo gain cannot be used to change the back emf calculation in the estimator model because the back emf is only a function of the torque constant.